1. Field of the Invention
This invention relates to semiconductor processing apparatus and more particularly to improved apparatus capable of transferring wafers within an evacuated environment, or atmospheric environment, or between both environments.
2. Brief Description of the Prior Art
Semiconductor wafers are typically processed in vacuum processing systems. These systems include one or more chambers, each performing wafer processing operations such as etching, chemical vapor deposition or physical vapor deposition, which often require heating or cooling of the wafer, and a plasma to assist the process. Typically the environment within such processing chambers is maintained at a low subatmospheric pressure. Each chamber includes inlets and outlets for evacuation apparatus and the admission of processing gases, as well as an aperture controlled by a slit valve to admit wafers. Such processing chambers may in turn communicate with a wafer transfer chamber, and in turn the transfer chamber will have a valve-controlled aperture by which wafers can be admitted from outside the system.
The transfer of a wafer to and from a chamber and to and from the outside of the system is generally done mechanically by means of a robot arm at the end of which is a wafer retaining means. There are two types of wafer retaining means used in the art. The first type is a flat blade through which a vacuum channel is formed, terminating in an outlet. This is so that the blade can pick up a wafer by touching the surface containing the outlet, typically the upper surface of the blade, to the bottom surface of the wafer and applying a vacuum, so as to cause the wafer to stick to the blade. The advantage of the flat vacuum blade pickup is that the blade, being flat and thin, can be relatively easily maneuvered between the tight spaces of a wafer storage cassette to pick up a wafer.
There are two prominent disadvantages to the vacuum blade. First, since the blade pickup function relies on vacuum suction to hold a wafer in place, the blade pickup is completely ineffective for holding a wafer in an evacuated environment. Second, the construction of the vacuum blade is expensive, and the blade can break down under high temperature or corrosive conditions. The construction of the blade is typically an expensive multilayer laminate of metal and plastic bonded together with silicone rubber. These layers tend to separate or warp when contacted with a hot wafer, and the blade can corrode in the presence of corrosive wafer processing gases.
To overcome the disadvantages of the flat vacuum blade, a shoe attached to the robot arm has been used. This second prior art wafer retaining surface comprises a shoe, or tray-like extension at the leading end of the arm, having a bevelled contour shaped to accommodate a wafer. The shoe helps to engage the wafer and retain the wafer in place upon the arm while the robot arm swings around to deliver the wafer to another location.
Although the shoe is fairly effective for moving wafers in a vacuum environment, it is less effective and efficient in the ambient atmospheric environment, particularly where wafers must be transferred to and from a standard wafer cassette. One drawback is that a vacuum pick on the shoe is typically still necessary to transfer wafers from a wafer cassette. In designs in which the arm to which the shoe is attached extends to the bottom center of the blade, only the front edge to the center of the shoe has the clearance necessary to extend into a standard wafer cassette. Such a shoe typically can extend only partially into the cassette, with the tip of the shoe adjacent the bottom center of a wafer. Accordingly a vacuum suction is applied to the wafer to insure positive retrieval and retention of the wafer on the shoe as the wafer is retracted from the cassette.
In order to permit the shoe to be extended even partially into the tight spacings of a standard wafer cassette, the shoe must be machined to be quite thin. But even a thin shoe inevitably is an undesireably close fit for the tight clearances within a standard wafer cassette, even if dimensioned to extend fully within a wafer cassette to engage a wafer. And simply relying on the thin retaining projection of such a thin shoe along with gravity to engage the wafer during transit may not provide sufficient assurance that wafers will be consistently retained.
Neither may such a thin shoe provide sufficient assurance that the wafer will assume a consistent position during transit. For example, when the inner edge of the shoe comes into contact with a wafer, the edge of the wafer at one position sometimes becomes caught in the interface between the inner edge of the shoe and the shoe bottom, so that the opposite position on the edge of the wafer is lifted upward. The wafer thus rests in a canted position in the shoe rather than centered and flat. When the wafer placement on the shoe is canted, wafer positioning on the processing support surface in the processing chamber is not consistent from wafer to wafer. This in turn can lead to a multitude of processing problems, such as non-uniformity, plasma arcing, and damage to the wafer and support surface.
Although the association of a vacuum pickup feature with such a shoe may help with such problems, it is at best only a partial solution. It does nothing to enhance the function of the shoe within evacuated environments, nor the inevitable clearance problem presented in accessing a standard cassette even with a thin shoe.
There is a need therefore for a wafer transfer apparatus and method which would be equally efficient and effective both in a vacuum and ambient environment, and in transferring wafers between both environments. Also very desirable would be a wafer transfer apparatus and method that consistently assures proper holding and centering of the wafer under every condition and environment encountered during wafer transferral. Likewise very desirable would be a wafer processing system which performs in an improved manner the transfer of wafers between a standard cassette and a transfer chamber at ambient pressure, as well as the transfer of wafers between a transfer chamber when evacuated to a subatmospheric pressure and one or more evacuated wafer processing chambers. It would also be very advantageous to provide a wafer transfer device that can withstand corrosive substances and elevated temperatures. The foregoing capabilities would be still more desirable if provided with a thin profile wafer transfer device with the capability of smoothly accessing between the tight spaces of standard wafer storage cassettes, and which incorporated both a vacuum conduit and capacitive sensors.